catwavez is 100% spot on. The M32 outputs BIdirectional CVs, where the vast majority of Eurorack uses UNIdirectional CV for control. This means that while the M32 is happy as a clam with negative CVs (ie: something like -1V is valid), Eurorack usually doesn't have the ability to make sense of negative CVs. So you either get fubar tunings when the M32's CV dips below 0V, or as in this case, the module simply won't respond to CVs below 0V at all. And yes, that's what the MScale is intended to correct.
Why does this exist? Simple...it's Moog. And don't EVEN get me started about their modulars' S-Trig idiocy, either!